Continental

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–3

  The snowpack currently holds multiple persistent weak interfaces, with the more likely issue being weak faceted snow in the upper snowpack. In wind-affected terrain, this weak snow is capped by old wind slabs, while in more sheltered areas it is buried by snowfall from 3/19–3/20. These weak interfaces may allow isolated trigger points to linger longer than normal, even though recent benign weather has allowed the snowpack to gradually settle. Strong winds over the last six weeks have created highly variable surface conditions, so there is not one consistent setup users can look for across the forecast area. The most likely depth of instability is in the upper 1–2 feet of the snowpack.

  This variability makes the snowpack difficult to assess and easy to misread. Users can reduce risk by choosing simple terrain and avoiding terrain traps.

  A second, lower-probability but higher-consequence concern is the 1/26 facet layer, which is buried around 2–3+ feet deep in many areas. We continue to identify this layer as widespread on all aspects and at elevations above 1000 feet. Stability tests continue to show an issue with decreasing sensitivity, but the possibility remains for a person to trigger a large avalanche. A human-triggered avalanche reported on Thompson Pass on 3/25 is a reminder of how unpredictable persistent weak layers can be. In that case, a slope was remotely triggered up to 4 feet deep after previously receiving heavy traffic from both ski tourers and snowmachiners. While the slope was relatively small, the event is a good reminder that large avalanches remain possible.

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Wet loose and other solar-triggered avalanche activity has remained limited so far, but is expected to become more of a concern as temperatures slowly return to seasonal norms. Calmer winds and a slight increase in temperatures this afternoon may allow sunny slopes, especially south- to west-facing terrain, to warm enough for small loose avalanches and other signs of instability to develop. Solar warming may also increase sensitivity at older weak interfaces in the upper snowpack, adding a new form of stress to a snowpack that has seen very little change in recent weeks.

  Watch for moistening surface snow, rollerballs, small loose avalanches, and failing cornices as slopes heat up through the day. If you begin to observe any of these signs, expect hazard to increase with additional sun exposure.

  Please share any observed avalanche activity to help us track how this problem is developing.

   

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2–3

  Weak snow near the ground is increasingly common as you move North through our forecast zones, with the Continental zone being the most concerning. Although it still remains unlikely to trigger this layer, areas that have been scoured from strong northeast winds now have a shallower snowpack, making this layer closer to the surface and potentially easier for a rider to trigger.

  This is a low-likelihood but high-consequence problem most likely to be triggered in steep, rocky terrain and areas where the snowpack is thin or scoured. Triggering this layer directly is unlikely at this time, but avalanches that start in the upper snowpack could step down to the basal weak layer.  There is a high degree of uncertainty regarding the poor structure at the base of the snowpack.